Turbines are widely used in industrial and commercial operations. A typical commercial steam or gas turbine used to generate electrical power includes alternating stages of stationary and rotating airfoils. For example, stationary vanes may be attached to a stationary component such as a casing that surrounds the turbine, and rotating blades may be attached to a rotor located along an axial centerline of the turbine. A compressed working fluid, such as but not limited to steam, combustion gases, or air, flows through the turbine, and the stationary vanes accelerate and direct the compressed working fluid onto the subsequent stage of rotating blades to impart motion to the rotating blades, thus turning the rotor and performing work.
An efficiency of the turbine generally increases with increased temperatures of the compressed working fluid. However, excessive temperatures within the turbine can reduce the longevity of the airfoils in the turbine and thus increase repairs, maintenance, and outages associated with the turbine. As a result, various designs and methods have been developed to provide cooling to the airfoils. For example, a cooling media can be supplied to a cavity inside the airfoil to convectively and/or conductively remove heat from the airfoil. In particular embodiments, the cooling media flows out of the cavity through cooling passages in the airfoil to provide film cooling over the outer surface of the airfoil.
As temperatures and/or performance standards continue to increase, the materials used for the airfoil become increasingly thin, making reliable manufacture of the airfoil increasingly difficult. For example, certain airfoils are cast from a high alloy metal, and a thermal barrier coating is applied to the outer surface of the airfoil to enhance thermal protection. A water jet can be used to create cooling passages through the thermal barrier coating and outer surface, but the water jet causes portions of the thermal barrier coating to chip off. Alternately, the thermal barrier coating can be applied to the outer surface of the airfoil after the cooling passages have been created by an electron discharge machine (EDM), but this requires additional processing to remove any thermal barrier coating covering the newly formed cooling passages. Moreover, this process of re-opening the cooling holes after the coating process becomes increasingly difficult and requires more labor hours and skill when the sizes of the cooling holes decrease and the number of cooling holes increase.
A laser drill utilizing a focused laser beam can be used to create the cooling passages through the airfoil with a reduced risk of chipping the thermal barrier coating. The laser drill, however, requires precise control due to the presence of the cavity within the airfoil. Once the laser drill breaks through a near wall of the airfoil, continued operation of the laser drill by conventional methods can result in damage to an opposite side of the cavity, potentially resulting in a damaged airfoil that must be refurbished or discarded.
Accordingly, a method and system for determining a breakthrough of a laser drill when drilling a hole in a component of a gas turbine would be beneficial. More particularly, a method and system for determining a breakthrough of a laser drill when drilling a hole in a component of a gas turbine based on one or more operating conditions determined during such a drilling process would be particularly useful.